Heat 3 cups of milk in the sauce pan over medium heat for regular pasteurized milk. (If using raw milk heat only to 110-115 degrees Fahrenheit to prevent the milk from cooking and losing important nutrients. I test the milk by sticking my (clean) finger into it after heating it on medium-low heat for several minutes and stirring. When the milk feels warm, but does not burn my finger, I remove it from the heat.)
Put 2-3 tablespoons of yogurt into a 1 quart mason jar and pour the warm milk into the jar. Stir the milk and yogurt together.
Wrap the mason jar in a towel and place it in an enclosed space like a microwave, slow cooker or cooler. Let the yogurt sit for 8-10 hours while the good bacteria grow and thicken the yogurt.
When the yogurt is done, depending on the type of starter used, it will be thicker than milk, but runnier than most commercial yogurt products. You can add maple syrup, honey or stevia as a sweetener. You can eat it plain or use it as a sauce. You can add fruit, nuts or other healthy goodies to individual servings.